The present invention relates to an optical disk medium for optically recording and reproducing information by using an optical disk device, and in particular to a phase change optical disk medium for recording information with a high density by using phase changes of a thin layer for recording information from its crystal phase to its amorphous phase and reproducing the information.
With development of the computer technology, it has become possible to process audio data and moving picture data by using a personal computer, besides text data processed conventionally. Largecapacity information storage devices capable of coping with various kinds of information media have become necessary. As an optical disk satisfying such a demand, there is a DVD (Digital Video Disk). A DVD-ROM dedicated for reproduction having a user capacity of 4.7 GB/surface has already been put into production. A rewritable DVD-RAM is also under development. As a draft of specifications of optical disks for DVD-RAM, "DVD Specifications for Rewritable Disc (DVD-RAM) Version 1.0; July 1997" is open to the public.
According to the draft of specifications of optical disks for DVD-RAM, a phase change recording layer on which information marks are recorded by phase changes from the crystal phase to the amorphous phase and vice versa is used as an information recording layer, with a capacity of 2.6 GB/surface, a track pitch of 0.74 .mu.m, and a minimum mark length of 0.614 .mu.m. Its track structure takes a shape of concave portions that alternate with convex portions. Each groove has a width nearly equal to that of each land portion. Using both groove and land portions as tracks, information marks can be recorded. In addition, cross-talk from adjacent tracks can be canceled. The so-called land/groove structure is thus adopted.
Furthermore, the draft of specifications of optical disks for DVD-RAM described that a test of disks should be conducted by recording and reproducing with a laser spot obtained by focusing a laser beam having a wavelength of approximately 650 nm by a lens having a numerical aperture of 0.6. A reproduced signal of a 14T mark having a mark length of approximately 2.87 .mu.m should have an amplitude equivalent to at least 43% of the reproduced signal level of unrecorded portions. A reproduced signal of a 3T mark having a mark length of approximately 0.614 .mu.m should have an amplitude equivalent to at least 40% of that of the reproduced signal of the 14T mark. Assuming that the reproduced signal level of the unrecorded portions is 100% in the case of a disk yielding the smallest reproduced signal, the amplitude of the reproduced signal of the 3T mark is 43%.times.40%.apprxeq.17%. If a phase change optical disk is capable of yielding at least 17% as the reproduced signal amplitude of the shortest 3T mark when a laser beam having a wavelength of approximately 650 nm and a lens having a numerical aperture of 0.6, information reproduction processing of the phase change optical disk can be conducted by using a drive for DVD-RAM.
For compiling and preserving data recorded on a DVD-ROM disk, a capacity nearly equivalent to that of the DVD-ROM disk or a capacity exceeding that is needed. However, the user capacity of an optical disk for DVD-RAM conforming to the above described draft of specifications is 2.6 GB/surface, which is approximately half of the capacity 4.7 GB/surface of the DVD-ROM. Therefore, the optical disk for DVD-RAM conforming to the draft of specifications is insufficient in capacity. It thus becomes difficult to compile and preserve various data including data recorded on the DVD-ROM disk.